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Outlook performance

asked 2019-12-09 16:47:21 +0000

quest4answer gravatar image

updated 2019-12-09 17:06:17 +0000

Hello: i have been troubleshooting issue with outlook slow latency with exchange server and noticed that load balancer is missing SACK option. attached are two traces - one from client and another from load balacner. is client and is lb.

any thoughts besides SACK missing from tcp handshake which constitute to slow outlook performance


here is the links to traces -loadbalacner - client

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Can you talk a little about the network topology?
In the client capture, the server is known by two different MAC addresses:

No. Source  Source  Destination Destination Info
1 f2:76:10:3d:cb:a3  f2:06:d9:43:0b:ce   53376 → 443 [SYN] Seq=0 Win=64240 Len=0 MSS=1460 WS=256 SACK_PERM=1
2  f2:cb:5f:89:23:87 f2:76:10:3d:cb:a3   443 → 53376 [SYN, ACK] Seq=0 Ack=1 Win=29200 Len=0 MSS=1460 WS=128

In the load balancer capture, the client comes is known by 5 difference MAC addresses.
Is that normal for this environment?

Chuckc gravatar imageChuckc ( 2019-12-09 22:29:56 +0000 )edit

i used Anonymizer so it might have changed the mac address

quest4answer gravatar imagequest4answer ( 2019-12-12 17:46:06 +0000 )edit

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answered 2019-12-09 18:32:15 +0000

SYN-bit gravatar image

Delayed ACK is adding another ~50ms delay per lost packet as each lost packet is retransmitted individually and therefor the ACK to each retransmission is sent a the DelayedACK timout (as there is not a second packet and the client does not have data to send).

So each lost packet takes ~168ms (iRTT) + ~50ms (DelayedAckTimeout) = ~218ms to be retransmitted. Multiplying by the about of packets lost (which seems to be happening in bursts) which is quite high.

If your loadbalancer supports it, you can enable SACK on the client-side and keep it disabled on the server side if your servers do not support it.

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That's a nice tcp graph. I agree, enabling SACK would be a huge improvement.

Christian_R gravatar imageChristian_R ( 2019-12-11 20:31:20 +0000 )edit

Okay .. sounds i am in right direction. i will enable sack at my next change window. thanks everyone

quest4answer gravatar imagequest4answer ( 2019-12-12 17:46:54 +0000 )edit

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Asked: 2019-12-09 16:47:21 +0000

Seen: 901 times

Last updated: Dec 09 '19